What Could Splitting Open Of Bump On Tailbone With Gushing Blood Mean?

i had fallen and hurt my tailbone i went to a doctor and they said nothing was wrong but they thought i might have broken it because of the pain it hurt to walk sit stand ect. well a few days later i felt a bumb on my tail bone and then a night later this lump split open and gushed out alot of old blood. what was the cause of this?